去年秋天，一家做光伏直流保护件的小厂把一叠资料发过来。
资料里有三份报告：一份 V-0、一份灼热丝、一份 CTI，都是第三方出的。
信里写了一句很客气的话："这三份都是合格的，为什么我们的件送安规还是被退回？"
我先把三份报告翻到测试条件那一栏，看到两个地方就停了。
CTI 那份，样品状态写的是干态；灼热丝那份，测的是1.5 毫米的标准试片，而这颗外壳最薄的壁只有 0.8 毫米。
我回了他一句：报告没问题，是报告回答的不是这颗件的问题。
他后来又补了一句："报告上写的是 600 伏。"
我回了三句：这颗件的分断能力等级是多少？长期工作温度多少？装的是室内柜还是户外箱？
三句问完，方向基本定了——问题不在阻燃够不够，在这颗件将来要在什么状态下挨着电跑十年。
这篇就把熔断器外壳材料这件事拆开讲：三个电气指标各管哪一段，几条路线各自站得住的地方，然后是判据表，最后说清哪些件不该走塑料这条路。
一、三个指标，其实是按"出事的时间点"分的
这三个指标最容易被混着用，因为它们都写在同一张资料页上。
换个角度看，它们负责的是三个不同的时间点。
电还没出事的时候，管这件事的是 CTI。
件长期带电，表面沾了污秽、又吸了潮气，两个电极之间慢慢爬出一条导电痕迹——这是漏电起痕。
它不出火、不冒烟，只是绝缘一点点失效。CTI 就是给这件事设的档。
局部开始过热的时候，管这件事的是灼热丝。
端子接触不良、长期过载，某个位置温度往上顶。GWIT 考的是这个温度顶到多少时材料会自己烧起来。
750℃、850℃、960℃ 是常见的三档，按件的等级要求取。
真的烧起来之后，才轮到 V-0。
火源移开以后材料能不能自己停——这是自熄性。它管的是"烧起来之后别蔓延"，管不了"会不会烧起来"。
所以这三份报告回答的是三个不同的问题。一份合格，不能替另外两份说话。
还有第四项容易被漏掉：耐电弧。
ASTM D495 测的是电弧灼烧下材料表面形成导电碳化路径的能力。熔断器在分断的瞬间，电弧能量是集中释放的——这一项在高分断等级的件上比 V-0 更贴现场。
这四项要分别要数据、分别验证。
一句话：熔断器外壳的电气要求，是一整套而不是一个等级。报一个 V-0 就定料，等于只回答了四分之一。
二、工况六维：这颗件被什么夹住
温度。分两层：长期工作温度常见 105 到 150℃（高分断件、密集排布的柜内更高），以及分断瞬间的短时高温。
分断那一下，电弧区域的温度是几千度量级，金属蒸气会把热量砸到外壳内壁上。这一下不是靠耐热挡住的，是靠结构和材料的碳化行为一起挡住的。
载荷与力学。外壳上有螺纹、卡扣、卡槽，保险丝座还有夹持结构。
长期受力的位置要按蠕变量级来算——塑料在 100℃ 以上长期受力，蠕变是绕不开的一项。
介质。户外的件要面对雨水、凝露、灰尘，光伏件还要面对长期直流与清洗剂。
寿命。电力类件的服役期常在 10 年以上，光伏件普遍按 25 年口径提要求。
外观与析出。这一项对塑料件尤其要紧：表面析出会同时影响外观和绝缘，两者都掉。
合规。V-0、GWIT 档位、CTI 档位、耐电弧、绝缘电阻、无卤要求，以及对应的 IEC 与 UL 体系。
六维里，温度和介质这两条最容易被低估，因为它们在样品阶段都不显现。
三、几条路线，各自站得住的地方
熔断器这一类件，传统上是陶瓷和热固性材料的地盘。塑料化的空间集中在低压外壳、座体和光伏类的结构件上。
| 路线 | 耐温量级 | 阻燃与电气的常见位置 | 在这个件上的定位 |
|---|
| 无卤阻燃 PA66-GF | 长期 120–140℃ | V-0 可做，GWIT 到 850℃ 档常见 | 中低压外壳与座体的常用路线 |
| 无卤阻燃 PA6T / PA9T | 长期 150℃ 以上 | V-0、GWIT 与 CTI 兼顾 | 高分断、薄壁、长期高温件 |
| 阻燃 PPA | 长期 140–150℃ | 尺寸与电气平衡 | 需要过炉或有薄壁要求的件 |
| 阻燃 PPS | 长期 150℃ 以上 | 电气与化学稳定好 | 耐化学与高湿场景；韧性与价格另评 |
| 阻燃 PBT | 长期 120℃ 量级 | CTI 表现好 | 电气件常见路线；韧性一般 |
| 陶瓷 / 热固性体系 | 更高 | 分断与耐弧表现好 | 高分断主回路外壳仍在用 |
不做"谁更好"的结论，只谈差在哪。
阻燃 PA66 的账是：性价比与工艺成熟度都占，是无卤体系里被走得最多的一条；代价是吸水与长期耐温，150℃ 以上要谨慎。
PA6T 与 PA9T 的账是：长期耐温和薄壁成型都更好，CTI 基线也高；代价是料温、模具和干燥成本一起上抬。
PPS 与 PBT 的账是：在电气件上各有成熟的位置，但落到熔断器这种要扛分断的件上，韧性和耐弧表现要单独验。
这里有一个绕不开的矛盾要单独说：阻燃和 CTI 是相互牵制的。
很多提高阻燃性的添加剂，本身会让材料的抗漏电起痕能力下降。
也就是说，"高阻燃 + 高 CTI"不是一个现成牌号的选择题，是一个配方与验证的顺序题。
一句提醒：换阻燃体系等于换一整套电气验证。别拿 A 体系的 CTI 报告去推 B 体系的件。
四、选型判据表（这一页值得收藏）
把上面的约束落成可核对的指标。下表的门限值是方向性建议，不是验收标准——实际数值必须由具体项目、具体工况和实测确定。
| 指标 | 方向性门限 | 验证方法 / 标准 | 常见失效 | 通行解法 | 对应助剂体系 |
|---|
| CTI | 按件电压等级定，常见 400 V / 600 V 档 | IEC 60112，判定状态必须写明 | 表面漏电起痕、绝缘下降 | 低吸湿基材 + 无卤阻燃 | 阻燃体系 |
| 灼热丝 | 按件等级取 750 / 850 / 960℃ | IEC 60695-2-12/13，按件最小壁厚 | 灼热丝起燃 | 阻燃体系 + 玻纤协同 | 阻燃体系 |
| 阻燃等级 | V-0，且必须按件最小壁厚报 | UL94 / IEC 60695 | 离火不自熄、滴落引燃 | 无卤阻燃体系 | 阻燃体系 |
| 耐电弧 | 按客户等级定 | ASTM D495 | 表面碳化形成导电通道 | 阻燃体系 + 降低碳化倾向 | 阻燃体系 |
| 长期热氧保留率 | 120℃×1000 h 后按件定 | ISO 527 | 件发白、脆化、夹持力衰减 | 稳定化体系 | 抗氧剂 |
| 表面析出 | 长期存放与湿热后表面无异常 | 件级湿热存放 + 表面观察与电阻复测 | 发白、发黏、绝缘下降 | 控制润滑与阻燃剂迁移 | 润滑剂（内外平衡） |
| 夹持力保持 | 座体长期受力后衰减按件核 | 件级插拔或保持力试验 | 接触电阻上升、发热 | 抗蠕变基材 + 结构补偿 | 材料本征决定 |
怎么用这张表：不要逐行打分。先看 CTI 和灼热丝这两行。
这两项过不去，后面的力学和外观做得再好，件也送不进安规——因为它们是送检的头一关，也是最容易在湿热之后掉档的一关。
一个提醒：表里"验证方法"这一列，CTI 的判定状态和灼热丝的测试厚度必须写进技术协议。
用干态数据去对户外件、用 1.5 毫米试片去对一个 0.8 毫米的壁，都能凑出合格报告，但答的不是这颗件的问题。
五、四条常见失效，和它们的真实根因
失效一：出厂检测全过，装到现场半年后绝缘电阻往下走。
这个现象的根因，排在前面的是析出。
表面析出这一条最难缠。
迁移到表面的那层东西很薄，微米量级，肉眼要在侧光下才看得见。但它足以把表面电阻拉低一截。
这就相当于给件穿了一层不太绝缘的外衣。平时没事，等凝露和灰尘一起来，CTI 的条件就凑齐了。
通行解法：先查润滑与阻燃体系的外迁移倾向，再查存放与安装环境的湿度。这一条比换基材有效。
这类件的时间线，通常是这个形状：
起点：样品按件报了 V-0、灼热丝和 CTI，三份都合格，项目顺利往下走。
潜伏：件在仓库里过了整个梅雨季，表面开始泛白，摸上去有点发涩，被当成灰尘。
爆发：装上柜子、凝露和灰尘一起来，绝缘电阻往下走，客户先怀疑的是接线工艺。
追溯：把留样拿出来，在湿热箱里过一轮再测表面电阻，两组数据放在一起就看清了。
结算：方案没换基材，换的是阻燃体系的析出倾向、脱模方式和仓储条件。
这条线里最容易被省掉的一步是"潜伏"——因为发白不报警，也不影响当时的任何一项检测。
失效二：同一批件，表面发白程度深浅不一。
根因在助剂侧：稳定化体系分散不均。
抗氧剂在混料阶段没混开，局部浓度高的地方老化慢、低的地方老化快，外观就先显出差别。
看到这个现象，先查混料工艺与母粒化，别急着换料。
失效三：分断试验之后，外壳内壁出现大面积的碳化坑。
根因要分两头看：一头是灭弧腔与泄压路径的结构设计，另一头是材料的抗碳化能力。
这里有一条要直说的：遇到分断表现不好，很常见的做法是把外壳壁加厚。
这个方向常常是反的——壁厚加上去，熔体量变大、冷却更不均匀、内应力更高，件反而更容易在冷热交替下开裂；而分断要解决的核心是电弧往哪走、气体往哪排，那是结构的事，不是壁厚的事。
失效四：保险丝座的夹持力一年后明显变小。
根因是蠕变。夹持结构长期受一个方向的分力，温度又在 100℃ 上下，材料会慢慢往受力方向让。
接触电阻跟着抬，接触位置跟着发热，发热又加快蠕变——这是一个会自己加速的循环。
通行解法：按件的长期温度选抗蠕变基材，同时用结构给弹簧或金属件留出补偿量，不要全靠塑料扛住那个力。
一句直说的：这类件的排查顺序是——先看表面状态与环境湿度，再看结构与受力，最后才怀疑牌号。
顺序倒过来，会把一个"析出加吸湿"的问题当成"料不行"处理掉。
六、加工与验证：几件必须提前定的事
干燥。PA66 与 PA6T、PA9T 都要严格干燥，必须用除湿干燥机。
含水率超标的料在高温下会水解降解，件会脆、会起银纹，而这两个现象在电气件上会被误判成阻燃体系的问题。
料温与滞留时间。阻燃体系的加工窗口通常比通用料窄。
料温过高、料筒里停得太久，都会让阻燃成分局部降解——降解之后既影响阻燃，也影响表面析出。
模温。薄壁电气件靠模温撑填充和表面质量。
模温低了表面粗糙，粗糙的表面更容易积污吸水，这一条会一路影响到将来的 CTI。
模具与脱模。电气件的脱模剂使用要克制，脱模剂残留本身就是一种表面污染物。
能靠脱模斜度和顶出结构解决的，不要靠多加润滑剂解决。
验证顺序。建议这样排，顺序不要换：
1. 材料级：按件最小壁厚做 V-0、灼热丝，按判定状态做 CTI
2. 件级电气：绝缘电阻、耐压、耐电弧
3. 长期老化：120℃ 热氧后复测电气与力学
4. 湿热与析出：湿热存放后复测表面电阻与外观
5. 分断与整机：按实际分断等级做分断试验，最后上整机
前一项不通过就往下走，后面的数据没有解释意义。
这里有个内行细节：电气件的 CTI，同一批件送检测之前先自己留一组做湿热预处理，两组一起比。
两组的差值，就是这个件对环境的敏感度。敏感度高的件，环境条件必须写进协议。
七、边界：哪些熔断器件不该走改性尼龙
这一段可能比前面六段更值钱。
其一，高分断等级的主回路外壳。
分断时释放的能量、金属蒸气和高速气流，对材料是复合冲击。这一档目前仍以陶瓷和热固性体系为主，塑料件通常落在低压与座体位置。
其二，长期工作温度高于 150℃ 的件。
这个温度之上，尼龙家族的长期性能保持要有数据支撑；没有数据就不要开这个头。
其三，长期浸泡在清洗剂或强溶剂里的件。
阻燃体系与基体在这种环境里都可能被抽出或溶胀，电气与力学一起掉。这类件要先做浸泡验证，不能靠推测。
其四，年用量不足以摊开安规验证成本的专用件。
这类件要报 V-0、灼热丝、CTI、耐电弧，还要跑长期老化与分断。每一项都是钱和时间，用量摊不开就不成立。
把这四条写在前面，不是劝退，是省时间。
电气件的开发周期本来就长，最贵的一种失败是"样品过了、批量过不了、安规复测卡住"——那时候模具已经开完、产线已经排完，回退的账比一开始大得多。
还有一句要说清：外壳和座体不是一件事。外壳更看阻燃、耐弧与分断表现；座体更看夹持力、蠕变和长期接触稳定性。这两类件可以同料，判据的排序不同。
换料风险清单（从通用 PA66-GF 换到无卤阻燃体系，要动的东西）
| 环节 | 要动什么 | 容易漏的点 |
|---|
| 配方体系 | 阻燃体系一换，电气与力学一起变 | 沿用原体系的验证结论 |
| 模具 | 收缩率与流动性变了，配合尺寸要复算 | 浇口与排气按原方案不动 |
| 干燥 | 换除湿干燥机，参数按实测含水率定 | 沿用通用料的干燥时间 |
| 料温与滞留 | 阻燃料窗口更窄，要控料筒停留 | 长时间停机后不洗料筒 |
| 脱模 | 减少脱模剂依赖，靠结构与斜度解决 | 多加润滑剂补偿脱模 |
| 表面与存放 | 件级包装与仓储湿度要定 | 按普通件的方式裸放 |
| 验证顺序 | 材料级 → 件级电气 → 长期老化 → 湿热析出 → 分断整机 | 前一项未过就往下走 |
一页纸汇报表（给要向上汇报的人）
| 场景 | 推荐路线 | 关键指标 | 验证标准 | 需先确认的条件 |
|---|
| 中低压外壳 | 无卤阻燃 PA66-GF | V-0、GWIT、CTI | 按件壁厚做 UL94 与 IEC 60695 | 件的等级与长期温度 |
| 薄壁、长期高温外壳 | 无卤阻燃 PA6T / PA9T | CTI、耐温、薄壁成型 | IEC 60112 + 长期热老化 | 加工窗口能否做到 |
| 保险丝座体 | 抗蠕变阻燃体系 | 夹持力保持、CTI | 件级保持力 + 湿热后电气 | 长期受力与温度 |
| 户外直流件 | 低吸湿阻燃体系 | 湿热后 CTI、绝缘电阻 | 湿热处理后测电气 | 安装环境与清洗方式 |
风险提示：本路线的主要不确定性在湿热与长期老化之后的电气保持，不在初始阻燃等级。
读者常问的三句
问：V-0 报告上写的是 1.5 毫米，我们的壁只有 0.8 毫米，这份报告还能用吗？
不能直接用。阻燃等级只在标注厚度及其以上成立，壁更薄要按实际壁厚重测，这是安规里最容易被忽略的一条。
问：CTI 能不能靠加厚爬电距离补？
距离解决的是路径长度，CTI 解决的是表面会不会被碳化。材料本身容易碳化，距离再长也只是把失效点挪个地方，两条要一起做。
问：无卤体系是不是一定比含卤好？
不是简单的优劣关系。无卤在烟气与部分电气表现上有它的位置，但流动性和加工窗口通常要付出代价。选哪一条要看件的等级要求和成型条件，不是看标签。
结语
回到开头那家做光伏直流保护件的客户。
我们后来做的事不复杂：按件的最小壁厚重做了灼热丝，按湿热带电的实际状态重做了 CTI，再把两批件的表面析出情况放到一起比。
重测之后，一项从 850℃ 掉到了 750℃ 档，CTI 从 600 伏档掉到了 400 伏档——跟他的现场表现对上了。
后来动的是三处：阻燃体系换成析出倾向更低的一支、脱模方式从多加润滑剂改成改模具斜度、包装与仓储加了湿度控制。
熔断器外壳材料的判断链，说到底只有三条：件的电压与分断等级定电气档位 → 长期温度定基材 → 湿热环境定表面与析出的余量。
三条定完，"这颗件能不能做塑料"这个问题自然就有答案了。
如果你手上正有一个外壳或保险丝座要定料，把三样东西发过来就能给方向：件的分断等级、长期工作温度、安装环境是室内柜还是户外箱。
「我们有 V-0 的报告，为什么安规还是过不了？」——最麻烦的询盘是这一句。选料这件事，越早问越省事。
我们做的事很具体：把 PA6、PA66、PA46、PA11、PA12、PA6T、PA9T 和尼龙合金这些树脂，改成某个件真正能用的样子；顺带做改性 PPO、PPS 和热塑性弹性体。
也经营各大化工巨头的尼龙树脂、副牌料和大包料现货。另：长期收尼龙原料、水口回料与各类尼龙废料，有正规处置渠道。
配方里的助剂体系按件的工况配——常规助剂常备现货，特殊型号按需配套；你报工况和牌号，料和助剂一次配齐。
这类件的选料与试模，可以一起聊。
Last autumn, a small factory that makes photovoltaic DC protection components sent over a stack of materials.
There are three reports in the materials: one V-0, one glow wire, and one CTI, all issued by third parties.
The letter contained a very polite sentence: 'All three of these are qualified, so why were our items sent to safety inspection still returned?'
I first flipped to the testing conditions column in the three reports and stopped when I saw two places.
For the CTI one, the sample state is written as dry; for the glowing wire one, what was measured was a 1.5 mm standard test piece, while the thinnest wall of this casing is only 0.8 mm.
I replied to him: The report is fine, it's the report that answers, not the question about this component.
He later added another sentence: 'The report says 600 volts.'
I replied with three sentences: What is the breaking capacity level of this component? What is the long-term operating temperature? Is it installed in an indoor cabinet or an outdoor box?
After asking three questions, the direction is basically set — the problem is not whether it is flame-retardant enough, but under what conditions this component will be next to electricity for ten years in the future.
This article will break down the issue of fuse casing materials: which segment each of the three electrical indicators covers, where the various lines each hold up, then the criteria table, and finally clarifying which parts should not use the plastic route.
1. The three indicators are actually divided according to the 'time of the incident'.
These three indicators are the easiest to be confused with each other because they are all written on the same information page.
From another perspective, they are responsible for three different points in time.
Before the electricity incident happened, the one in charge of this matter was CTI.
When the component is charged for a long time, its surface gets dirty and absorbs moisture, and a conductive trace slowly creeps between the two electrodes—this is tracking due to leakage.
It doesn't spark or smoke; it's just that the insulation slightly fails. CTI is the rating set for this issue.
When the local area starts to overheat, the thing dealing with it is the hot wire.
Poor terminal contact and long-term overload cause the temperature at a certain spot to rise. GWIT tests how high the temperature can get before the material ignites by itself.
750℃, 850℃, and 960℃ are the three common levels, selected according to the grade requirements per piece.
It’s only after it really catches fire that V-0 comes into play.
Whether the material can stop burning on its own after the ignition source is removed - this is self-extinguishing. It concerns 'doesn't spread after catching fire', not 'whether it will catch fire'.
So these three reports answer three different questions. One is qualified, but it cannot speak for the other two.
There is also a fourth item that is easy to overlook: arc resistance.
ASTM D495 tests the ability of a material to form a conductive carbonization path on its surface under arc burning. At the moment the fuse breaks, the arc energy is released in a concentrated manner—this aspect is more practical for high interrupting rating components than V-0.
These four items each require data and separate verification.
In one sentence: the electrical requirements for the fuse housing are a complete set, not just a single rating. Reporting V-0 as the material specification only answers a quarter of the question.
2. Working condition six dimensions: What is clamping this part?
Temperature. It is divided into two levels: the common long-term operating temperature is 105 to 150°C (higher for high-breaking components and densely arranged cabinets), and the short-term high temperature during breaking moments.
At the moment of cutting, the temperature in the arc zone reaches several thousand degrees, and metal vapor will transfer the heat onto the inner wall of the casing. This is not blocked by heat resistance alone, but by the combination of structural design and the carbonization behavior of the material.
Load and mechanics. There are threads, clips, and slots on the shell, and the fuse holder also has a clamping structure.
The position under long-term stress should be calculated according to the creep strain level—plastic under long-term stress above 100°C inevitably experiences creep.
Medium. Outdoor components have to face rain, condensation, and dust, and photovoltaic components also have to face long-term direct current and cleaning agents.
Service life. The service period of electrical components is often over 10 years, while photovoltaic components are generally required according to a 25-year standard.
Appearance and exudation. This item is particularly important for plastic parts: surface exudation will affect both appearance and insulation, and both will be compromised.
Compliance. V-0, GWIT level, CTI level, arc resistance, insulation resistance, halogen-free requirements, and the corresponding IEC and UL systems.
Among the six dimensions, temperature and medium are the two most easily underestimated, because they do not appear during the sample stage.
3. Several routes, each with its own merits
Fuses are traditionally the realm of ceramics and thermosetting materials. The use of plastics is concentrated on low-voltage enclosures, bases, and structural components in the photovoltaic sector.
| Route | Temperature tolerance level | Common Locations of Flame Retardant and Electrical | The positioning on this piece |
|---|
| Halogen-free flame retardant PA66-GF | Long-term 120–140℃ | V-0 is doable, GWIT commonly reaches up to 850℃ | Common routes for medium and low voltage enclosures and bases |
| Halogen-free flame-retardant PA6T / PA9T | Above 150°C for a long period | V-0, taking both GWIT and CTI into account | High scoring fracture, thin-walled, long-term high-temperature components |
| Flame-retardant PPA | Long-term 140–150℃ | Size and Electrical Balance | Parts that require heat treatment or have thin-wall requirements |
| Flame-retardant PPS | Long-term temperatures above 150°C | Good electrical and chemical stability | Resistant to chemicals and high humidity environments; toughness and price to be evaluated separately |
| Flame-retardant PBT | Long-term 120℃ magnitude | CTI performs well | Common routes for electrical components; average toughness |
| Ceramic / Thermosetting System | Higher | Good breaking and arc resistance performance | The high-voltage main circuit casing is still in use |
Don't make a 'who is better' conclusion, just talk about the differences.
The story of flame-retardant PA66 is: it ranks high in both cost-performance and process maturity, and it is the most widely used in halogen-free systems; the price to pay is water absorption and long-term heat resistance, so caution is needed above 150°C.
The situation with PA6T and PA9T is: they are better in terms of long-term heat resistance and thin-wall molding, and their CTI baseline is also higher; the cost is that the material temperature, molds, and drying costs all go up together.
The situation with PPS and PBT is: both have established uses in electrical components, but when it comes to fuses, which need to withstand breaking currents, toughness and arc resistance must be tested separately.
There is an unavoidable contradiction that needs to be discussed separately: flame retardancy and CTI are mutually constraining.
Many flame-retardant additives themselves can reduce the material's resistance to tracking and leakage.
In other words, 'high flame retardancy, high CTI' is not a multiple-choice question of ready-made grades; it is a sequential question of formulation and validation.
A reminder: changing the flame-retardant system is equivalent to changing the entire electrical verification. Don't use the CTI report of system A to apply to components of system B.
4. Selection Criteria Table (This page is worth keeping)
Translate the above constraints into verifiable indicators. The threshold values in the table are directional suggestions, not acceptance criteria—the actual values must be determined by specific projects, specific working conditions, and actual measurements.
| Indicator | Directional Threshold | Verification Method / Standard | Common Failures | Common solution | Corresponding auxiliary agent system |
|---|
| CTI | Determined by the voltage rating per item, commonly 400 V / 600 V range | IEC 60112, the determination status must be specified | Surface tracking and insulation degradation | Low moisture-absorption substrate, halogen-free flame retardant | Flame retardant system |
| Scorching thread | Based on item grade, take 750 / 850 / 960°C | IEC 60695-2-12/13, minimum wall thickness per piece | The hot wire ignites | Flame-retardant system with glass fiber synergy | Flame retardant system |
| Flame retardant rating | V-0, and must be reported according to the minimum wall thickness per item | UL94 / IEC 60695 | The fire does not extinguish by itself; dripping causes ignition. | Halogen-free flame retardant system | Flame retardant system |
| Arc-resistant | Based on customer level | ASTM D495 | Surface carbonization forms a conductive channel | Flame-retardant system reduces charring tendency | Flame retardant system |
| Long-term thermal-oxygen retention rate | After 120℃ × 1000 h, determine per item | ISO 527 | Whitening of parts, embrittlement, and reduction in clamping force | Stabilization system | Antioxidant |
| Surface precipitation | After long-term storage in humid and hot conditions, the surface shows no abnormalities | Component-level damp heat storage Surface observation and resistance remeasurement | Fading, sticky, insulation degradation | Control the migration of lubricants and flame retardants | Lubricant (Internal and External Balance) |
| Clamping force retention | Seat body long-term stress decay based on component core | Component-level plug-in or holding force test | Increase in contact resistance and heating | Creep-resistant substrate Structural compensation | Intrinsic determination of materials |
How to use this table: Do not score row by row. First, look at the CTI and the Incandescent Wire rows.
If these two items cannot pass, no matter how well the later mechanics and appearance are done, the parts cannot be sent for safety certification—they are the first checkpoint for inspection, and also the one most likely to fail after being exposed to heat and humidity.
A reminder: In the 'Verification Method' column inside and outside the table, CTI judgment status and the test thickness of the glow wire must be included in the technical protocol.
Using dry-state data for outdoor parts, or using a 1.5 mm test piece for a 0.8 mm wall, can both result in a passing report, but it doesn't address the problem of the actual part.
Five, four common failures and their real root causes
Failure 1: All factory tests passed, but six months after installation on-site, the insulation resistance starts to decrease.
The root cause of this phenomenon, at the forefront, is precipitation.
Surface precipitation is the most troublesome.
The layer that migrates to the surface is very thin, on the order of microns, and can only be seen with the naked eye under side lighting. But it is enough to lower the surface resistance by a notch.
This is equivalent to giving the component a layer of not very insulating jacket. Normally it's fine, but when condensation and dust come together, the CTI conditions are met.
Common approach: First check the migration tendency of the lubrication and flame-retardant system, then check the humidity of the storage and installation environment. This is more effective than changing the base material.
The timeline for this kind of piece is usually in this shape:
Starting point: The samples were tested for V-0, glow wire, and CTI per piece, all three passed, and the project proceeded smoothly.
Lurking: The item stayed in the warehouse throughout the entire plum rain season, its surface began to turn white, it felt a bit rough to the touch, and it was mistaken for dust.
Outbreak: When the cabinet is installed, along with condensation and dust, the insulation resistance drops, and the customer's first suspicion is the wiring craftsmanship.
Trace back: Take out the sample, put it through a cycle in the hot and humid chamber, and then measure the surface resistance. When the two sets of data are put together, it becomes clear.
Conclusion: The plan did not change the base material, but changed the precipitation tendency of the flame-retardant system, the demolding method, and the storage conditions.
The step most easily skipped in this process is 'incubation' — because turning pale does not trigger any alarms and does not affect any of the tests at the time.
Failure 2: For the same batch of items, the degree of surface whitening varies.
The root cause lies on the additive side: the stabilization system is unevenly dispersed.
If the antioxidant is not properly mixed during the blending stage, areas with high local concentrations will age slowly while areas with low concentrations will age quickly, and the differences will first appear in the appearance.
Seeing this phenomenon, first check the mixing process and masterbatching, don't rush to change the material.
Failure 3: After the断 test, large areas of carbonization pits appeared on the inner wall of the shell.
The root cause should be looked at from two perspectives: one is the structural design of the arc-extinguishing chamber and pressure relief path, and the other is the material's resistance to carbonation.
Here's something that needs to be said directly: when the segmentation performs poorly, a very common approach is to thicken the shell walls.
This direction is often the opposite—when the wall thickness is increased, the melt volume increases, cooling becomes more uneven, internal stress is higher, and the part is actually more prone to cracking under alternating hot and cold conditions; the core issue that fragmentation needs to solve is where the arc goes and where the gas is discharged, which is a structural matter, not a matter of wall thickness.
Failure Four: The clamping force of the fuse holder becomes significantly weaker after one year.
The root cause is creep. When a clamping structure is subjected to a directional force for a long time and the temperature is around 100°C, the material will slowly yield in the direction of the force.
The contact resistance rises, the contact point heats up, and the heating accelerates creep - this is a self-accelerating cycle.
Common solution: Select creep-resistant materials according to the long-term temperature of the part, and at the same time, provide compensation in the structure for springs or metal parts, rather than relying entirely on the plastic to withstand that force.
A straightforward way to say it: the inspection order for this type of part is—first check the surface condition and environmental humidity, then look at the structure and stress, and only finally suspect the grade.
If the order is reversed, a problem of 'precipitation plus moisture absorption' will be treated as 'material is not good'.
6. Processing and Verification: Several Things That Must Be Decided in Advance
Drying. PA66, as well as PA6T and PA9T, must be thoroughly dried and must be dried using a dehumidifying dryer.
Materials with excessive moisture content will hydrolyze and degrade at high temperatures, causing the parts to become brittle and develop silver streaks, and these two phenomena on electrical components can be mistakenly judged as issues with the flame-retardant system.
Material temperature and residence time. The processing window of flame-retardant systems is usually narrower than that of general materials.
If the material temperature is too high or it stays in the barrel for too long, the flame retardant components will locally degrade — after degradation, it affects both flame retardancy and surface precipitation.
Mold temperature. Thin-walled electrical parts rely on mold temperature to support filling and surface quality.
If the mold temperature is low, the surface will be rough, and a rough surface is more likely to accumulate dirt and absorb water. This will continuously affect the future CTI.
Molds and demolding. The use of release agents on electrical components should be restrained, as the residue of release agents itself is a type of surface contaminant.
If the solution can be solved by the demolding angle and ejection structure, do not rely on adding extra lubricant.
Verification order. It is recommended to arrange it like this; do not change the order:
1. Material level: Conduct V-0 and glowing wire tests based on the minimum wall thickness per part, and perform CTI tests according to the determined condition.
2. Component-level electrical: insulation resistance, withstand voltage, arc resistance
3. Long-term aging: Re-test electrical and mechanical properties after 120°C hot oxygen exposure
4. Damp heat and precipitation: Re-measure surface resistance and appearance after damp heat storage
5. Separation and complete machine: Conduct separation tests according to the actual separation level, and finally assemble the complete machine
If the previous item fails, just move on; the subsequent data has no explanatory value.
Here is an insider detail: For the CTI of electrical components, keep a set from the same batch for damp heat pre-treatment before sending the batch for testing, and then compare the two sets together.
The difference between the two groups is the sensitivity of the component to the environment. For components with high sensitivity, environmental conditions must be included in the protocol.
7. Boundaries: Which fuse components should not use modified nylon
This section might be more valuable than the previous six sections.
First, the main circuit enclosure with a high short-circuit rating.
The energy, metal vapor, and high-speed gas released during fragmentation create a compound impact on the material. Currently, this category is still mainly composed of ceramics and thermosetting systems, while plastic parts usually fall into the low-pressure and seat positions.
Secondly, parts with a long-term operating temperature above 150°C.
Above this temperature, the long-term performance retention of the nylon family needs to be supported by data; without data, do not start this.
Third, parts that have been soaked in cleaning agents or strong solvents for a long time.
Both the flame-retardant system and the matrix may be leached or swollen in this kind of environment, causing both electrical and mechanical properties to deteriorate. Components of this type must undergo soaking verification first and cannot rely on assumptions.
Fourth, specialized parts whose annual usage is insufficient to spread out the cost of safety regulation verification.
These kinds of parts need to meet V-0, glow wire, CTI, and arc resistance standards, and they also need to undergo long-term aging and switching tests. Each item costs money and time, and if the quantity is too small to spread the cost, it won't be feasible.
Writing these four points first is not to discourage, but to save time.
The development cycle of electrical components is inherently long, and the most expensive type of failure is 'samples pass, mass production fails, and safety re-testing gets stuck' — by that time, the molds have been completed and the production line has been scheduled, so the cost of rolling back is much greater than at the beginning.
There is one more thing to clarify: the outer casing and the base are not the same thing. The casing focuses more on flame resistance, arc resistance, and breaking performance; the base focuses more on clamping force, creep, and long-term contact stability. These two types of parts can be made of the same material, but the criteria are prioritized differently.
Material Change Risk List (Things that need to be changed when switching from general PA66-GF to a halogen-free flame-retardant system)
| link; segment; part | What do you want to move? | Points that are easy to overlook |
|---|
| Formulation System | Once the flame-retardant system is changed, both electrical and mechanical aspects change together | Carry over the verification conclusions of the original system |
| Mold | The shrinkage rate and fluidity have changed, and the matching dimensions need to be recalculated. | The gate and vent remain unchanged according to the original plan. |
| Dry | Replace the dehumidifying dryer, and set the parameters according to the measured moisture content. | Use the drying time of the general-purpose material |
| Material Temperature and Residence Time | The flame-retardant material window is narrower, and the material barrel dwell needs to be controlled | Not cleaning the barrel after a long shutdown |
| Demolding | Reduce reliance on release agents, solve it through structure and inclination | Add more lubricant to compensate for demolding |
| Surface and Storage | The humidity for unit packaging and storage needs to be specified | Place it bare in the normal way |
| Verification order | Material level → Component level electrical → Long-term aging → Damp heat precipitation → Complete machine breakdown | If the previous item fails, just move on. |
One-page report sheet (for people who need to report upwards)
| Scene | Recommended Route | Key indicators | Verification Standard | Conditions that need to be confirmed first |
|---|
| Medium and low voltage enclosure | Halogen-free flame retardant PA66-GF | V-0, GWIT, CTI | Perform UL94 and IEC 60695 according to the part's wall thickness | Component grade and long-term temperature |
| Thin-walled, long-term high-temperature shell | Halogen-free flame-retardant PA6T / PA9T | CTI, temperature resistance, thin-wall molding | IEC 60112 Long-term thermal aging | Can the processing window be achieved? |
| Fuse holder body | Creep-resistant flame-retardant system | Clamping force retention, CTI | Component-level retention after damp heat electrical | Long-term stress and temperature |
| Outdoor DC components | Low moisture-absorbing flame-retardant system | CTI and insulation resistance after damp heat | Measure electricity after damp-heat treatment | Installation Environment and Cleaning Methods |
Risk warning: The main uncertainty of this route lies in electrical retention after humidity, heat, and long-term aging, not in the initial flame retardant rating.
Three questions readers often ask
Question: The V-0 report states 1.5 millimeters, but our wall is only 0.8 millimeters. Can this report still be used?
Cannot be used directly. The flame retardant rating only applies at the specified thickness and above; if the wall is thinner, it must be retested according to the actual wall thickness. This is one of the most easily overlooked points in safety regulations.
Question: Can CTI be compensated by increasing the creepage distance?
Distance addresses the path length, while CTI addresses whether the surface will carbonize. If the material itself is prone to carbonization, increasing the distance only shifts the failure point; both need to be considered together.
Question: Is a halogen-free system necessarily better than a halogen-containing one?
It’s not simply a matter of better or worse. Halogen-free has its place in smoke and some electrical performance, but flowability and processing window usually come at a cost. Which one to choose depends on the part’s grade requirements and molding conditions, not the label.
Conclusion
Returning to the client at the beginning who makes photovoltaic DC protection components.
What we did later wasn't complicated: we remade the heating wire according to the minimum wall thickness of the parts, remade the CTI according to the actual conditions of wet and hot live operation, and then compared the surface precipitation of the two batches of parts.
After retesting, one parameter dropped from 850℃ to 750℃, and the CTI dropped from 600 V to 400 V—matching his on-site performance.
Later, three changes were made: the flame retardant system was replaced with one less prone to precipitation, the demolding method was changed from using extra lubricant to changing the mold angle, and humidity control was added to packaging and storage.
The judgment chain for fuse housing materials ultimately boils down to three things: the electrical grade is defined by the part's voltage and breaking rating → the long-term temperature determines the base material → the wet and hot environment determines the surface and residual precipitation.
Once these three are determined, the question of "whether this part can be made of plastic" naturally has an answer.
If you currently have a housing or fuse holder and need to determine the material, just provide these three pieces of information and you can get a direction: part's breaking rating, long-term operating temperature, and whether the installation environment is an indoor cabinet or an outdoor box.
"We have a V-0 report, so why can't we pass safety certification?" — the most troublesome inquiry is this sentence. The earlier you ask about material selection, the easier it is.
We work on very specific tasks: turning PA6, PA66, PA46, PA11, PA12, PA6T, PA9T, and nylon alloys into a form that a particular part can actually use; we also modify PPO, PPS, and thermoplastic elastomers.
We also handle nylon resins, secondary brand materials, and bulk materials from major chemical companies. Additionally, we have long-term channels for acquiring nylon raw materials, sprue regrind, and various nylon wastes, with proper disposal routes.
The additive system in the formulation is configured according to the part's working conditions—conventional additives are readily available, special grades are tailored as needed; you provide the working conditions and grade, and both material and additives are supplied in one go.
Material selection and mold trial for these parts can be discussed together.
